A very wide range of devices have been developed to permit safe lifting of very large structures during manufacture or installation, such as large aircraft subassemblies, ship hull panels, large machines or machine components, etc. These devices range from simple eyebolts through shackles to hoist rings and custom clamps. Extreme care must be used in selecting and using a hoisting device for a particular structure. Breaking a lifting device through misuse or overloading can allow a multi-ton structure to fall, with severe damage to the structure and the surrounding area, in addition to possible severe injuries to workers.
Simple eye bolts and similar connections are only suitable with a lift line lying precisely along the length of the bolt. Any side loading is likely to result in catastrophic failure of the bolt. Because of this, shackles that pivot in one plane are used where the load direction may vary along a single plane. Shackles are often used with chain or rigging lines to attach fittings or to suspend a line. Where the load direction will vary in one plane, a hinged hoist ring may be used to accommodate the changing load direction. Typical of such hoist rings is that disclosed in U.S. Pat. No. 4,557,513. These hoist rings cannot, however, accommodate loads from any direction other than along the hinging plane.
When large, irregularly shaped structures such as aircraft panels or ship hull panels must be lifted, the lift load line will be in a direction well away from the fitting attachment bolt centerline and may vary in several different directions during the lift. Fully swiveling hoist rings, typically having rotating bases with a hinged ring, are used in such applications. Typical of such swiveling rings are those described in U.S. Pat. Nos. 3,297,293 and 4,705,422. These rings are very useful within their lift weight ratings.
Present swiveling lift rings, such as those described in the mentioned patents, tend to be complex and heavy and to have relatively low safe-load ratings. The separately installed shoulder pins and rings used as a unit often do not have the required load rating. Most are fastened to the structure to be lifted by a single bolt. In some applications it is desirable, as an additional safety measure, to attach the hoist ring assembly to the structure by two or more bolts while retaining the full swiveling and pivoting capability. A hoist ring assembly must fully and safely satisfy load and stress strength requirements from various angles and directions and under often adverse conditions.
Thus, there is a continuing need for improved hoist assemblies that are more versatile, have higher load ratings without excessive weight, will not accidentally detach, can be installed more quickly and are simple and inexpensive to manufacture and use.